1. Field of the Invention
This invention relates generally to a method of terminating ethylene polymerization and copolymerization processes and, more specifically, this invention relates to an improvement in the deactivation of Ziegler catalysts used in such polymerization processes.
2. Description of the Prior Art
In polyethylene production generally, and especially in the production of high density polyethylene (HDPE) or linear low density polyethylene copolymers (LLDPE) in high pressure reactors, it is necessary to terminate the polymerization reaction at a desired point in order to prevent the production of undesirable products, such as waxes and low molecular weight materials such as oligomers.
Typically in such processes, a transition metal derivative-containing catalyst (conventionally referred to as a "Ziegler catalyst") in combination with an organometallic cocatalyst is contacted with ethylene, or ethylene and one or more 1-olefin or diolefin comonomers having at least three carbon atoms, under conditions of moderate to high pressure and high temperature. Under such conditions, ethylene is a liquid and acts as a solvent for the product polymer.
Typically, the product polymer is discharged from the reactor in the form of a molten polymer solution stream into one or more relatively low pressure separators where gaseous monomer and comonomer are removed from the polymer.
The molten polymer solution stream actively polymerizes as it leaves the reactor, since catalyst components remain in the stream. Thus, it is necessary to terminate the polymerization reaction by deactivating the catalyst components in order to avoid the production of undesirable by-products.
One prior method of terminating ethylene polymerization reactions is disclosed in U.S. Pat. No. 4,105,609 (Machon et al) issued Aug. 8, 1978. The Machon et al patent describes the use of an alkali metal salt or alkaline earth metal salt of a carboxylic acid, such as calcium stearate, as a deactivating agent for a Ziegler catalyst.
Other prior art deactivation methods utilize such deactivating agents as humidified calcium stearate, salts of alkali metals, inorganic polybasic acids plus water, or water combined with an alkaline earth metal or zinc salt of an aliphatic monocarboxylic acid of 6 to 22 carbon atoms. Other prior deactivating agents include aqueous nonionic surfactants and flux-calcined diatomite.
None of the prior methods of catalyst deactivation have been completely satisfactory in terminating ethylene polymerization and copolymerization processes.